X-ray differential phase contrast imaging (dPCI) visualizes the phase information of coherent X-rays passing through a scanned object. For example, the coherent X-rays may be generated by a source grating between an incoherent X-ray source and the scanned object. In addition to classical X-ray attenuation imaging, dPCI may determine not only the absorption properties of the object along a projection line, but also the phase-shift of the transmitted X-rays. After the object, a phase-shifting grating (also known as phase grating) is placed, which generates an interference pattern that contains the required information about the beam phase-shift in the relative position of its minima and maxima, typically in the order of several micrometers. Since a common X-ray detector may not be able to resolve such fine structures, the interference pattern is sampled with an analyzer grating (also known as absorber grating), which features a periodic pattern of transmitting an absorbing strip with a periodicity similar to that of the interference pattern. The similar periodicity produces a Moiré pattern behind the grating with a much larger periodicity, which is detectable by a common X-ray detector.
For performing dPCI there exist essentially two different system geometries: planar 2D detection and slit-scanning systems.
In planar 2D detection, a 2D detector array takes an entire projection image in a single X-ray exposure and the phase acquisition has to be realized by a process called “phase-stepping” with, for example, 4, 8 or 16 exposures, in which one of the source grating, the phase grating and the absorber grating is moved relative to the other two gratings.
In the slit scanning approach, the woman's breast is scanned by a scan arm or gantry movement below the breast. The redundancy of the data acquisition by means of a typical arrangement of a number of parallel detector lines can be exploited to eliminate the need for phase-stepping and the gratings need not be moved with respect to each other. Hence, the phase-acquisition can be implemented in the ordinary scanning motion, as, for example described in WO2013/004574A1.
However, in both cases, the differential phase contrast technique, characterized by its use of three gratings, has the disadvantage that approximately 50% of the X-rays that have passed through the breast tissue are actually absorbed by the absorber grating and thus are lost for imaging or are not used for imaging.
Sometimes it may be desirable to perform not only dPCI imaging but instead or in addition attenuation imaging without gratings. For example, W2012/0099702 A1 shows a differential phase contrast imaging device with a movable grating.
Calibration of the detector and dPCI system may be difficult, if the gratings cannot be removed, and calibration e.g. for drift or other detector performance changes may be required more than just at the factory/initial installation but on a regular basis, and at least after every maintenance.
WO 2013/111050 A1 discloses an x-ray system provided with grating structures, such grating structures comprising sets of slits, which sets have mutually different directions.
U.S. 2013/0202081 A1 discloses a detector arrangement for phase contrast imaging comprising movable gratings.
US 2013/0230135 A1 discloses a joint imaging apparatus comprising gratings.
WO 2011/070488 A1 discloses a device for phase contrast imaging comprising a grating being movable out of the X-ray beam.